The high efficiency, low capital cost and short lead time of gas turbine based systems make them particularly attractive to electric utilities as a means for producing electrical power. However, traditionally, gas turbine operation has been limited to expensive, sometimes scarce, fuels--chiefly distillate oil and natural gas. As a result of the ready availability and low cost of coal, considerable effort has been expended toward developing a gas turbine system for generating electrical power which can utilize coal as its primary fuel.
Unfortunately, burning coal directly in the hot gas which is to flow through the turbine section of the gas turbine presents a variety of problems, such as erosion of the turbine blades due to particulate matter entrained in the gas and corrosion due to various contaminates in the coal. Consequently, interest has focused on indirect heating of the gas to be expanded in the turbine section by flowing the compressed air discharging from the compression section of the gas turbine through a heat exchanger disposed in a coal fired furnace, such as a fluidized bed combustor. However, such heat exchangers are incapable of heating the air entering the turbine to the temperatures required for high efficiency in modern turbines.
Consequently, it has been proposed to employ a topping combustor burning a conventional gas turbine fuel (i e., natural gas or no. 2 distillate oil) to further heat the gas from the heat exchanger. Such a scheme is shown in U.S. Pat. No. 4,369,624, (Hamm et al.), assigned to the same assignee as the current invention and hereby incorporated by reference.
Unfortunately, conventional gas turbine combustors are not suitable for use as topping combustors for several reasons. First, they rely on combustion air for cooling. In conventional gas turbines this presents no problem since the combustion air is the compressed air discharged by the compressor section. The temperatures of such air is typically only 370.degree. C. (700.degree. F.) and, hence, is suitable for cooling purposes. However, topping combustors use air from the coal furnace heat exchanger as combustion air and the temperature of such air must be relatively high to obtain high efficiency, typically 980.degree. C. (1800.degree. F.). As a result, a conventional gas turbine combustor would be insufficiently cooled to achieve adequate durability when used as a topping combustor. Second, the high temperature of the combustion air supplied to the topping combustor promotes the formation of nitrogen oxides (NO.sub.x), a harmful air pollutant. Hence, the amount of NO.sub.x produced by a conventional gas turbine combustor supplied with such high temperature combustion air would be objectionable.
Consequently, it would be desirable to provide a topping combustor for an indirect fired gas turbine which employs hot compressed air from a heat exchanger as combustion air yet which achieves adequate cooling for good durability and which minimizes the formation of NO.sub.x.